This invention relates generally to transfer apparatus, and more particularly to an electrostatic transfer apparatus including a transfer roller and a compliant back-up roller.
In a typical electrographic process for making reproductions, an electrostatic charge pattern, having an image-wise configuration corresponding to information to be reproduced, is formed on the surface of a grounded dielectric member. The charge pattern is developed by applying electroscopic developer material to such pattern to form a transferable image on the member. The developer material includes, for example, thermoplastic pigmented electroscopic marking particles which are attracted to the charge pattern and held on the member by electrostatic forces. The transferable image is transferred from the dielectric member to a receiver member, and then permanently fixed to such receiver member to form the reproduction.
Transfer is accomplished, for example, by electrostatically charging the receiver member to a level sufficient to attract the developer material from the dielectric member to the receiver member while the receiver member is in contact with the area of the dielectric member carrying the transferable image. Electrical charging of the receiver member is commonly effected by ion emission (for example, from a corona charger) onto a surface of the receiver member, or by contacting the surface of the receiver member opposite the dielectric member with an electrically biased transfer roller.
An electrically biased transfer roller is particularly suitable for use in an electrographic process where multiple related images are sequentially transferred in superimposed relation to a receiver member to form a composite reproduction, such as in making a multi-color reproduction. In such process, the receiver member is attached (tacked) to the transfer roller, and such member is successively returned into registered contact with the related transferable images on the dielectric member for superimposed transfer of such images to the receiver member. Examples of an electrically biased transfer roller are shown in U.S. Pat. No. 3,633,543, issued Jan. 11, 1972, in the name of Pitasi et al, and U.S. Pat. No. 3,832,055, issued Aug. 27, 1974, in the name of Hamaker.
In the above-mentioned patents, the dielectric member is a continuous web. Any adjustments necessary to register successive transferable images on the web with the receiver member to which such images are to be transferred, requires relative movement between the transfer roller (supporting the receiver member) and the web while the two are in contact. Such relative movement can damage the web or the transfer roller. In copending U.S. Pat. No. 4,436,405, issued Mar. 13, 1984, in the name of Kindt, a multi-color reproduction apparatus is described in which the dielectric member is a plurality of discrete sheets on which related transferable images are respectively formed. This apparatus has certain advantages over apparatus utilizing a continuous dielectric web. Specifically, the transport speed of discrete sheets through electrographic process stations can be regulated to optimize the respective functions of such stations, and registration of the transferable images on the discrete sheets relative to the receiver member can be accomplished prior to the two being brought into contact for transfer. However the transfer mechanism described in the above-mentioned Kindt patent, for assuring registration between the successive film sheets bearing related transferable images and the receiver member to which the images are transferred, is complex in nature and provides only a small nip area over which transfer occurs. Such small nip area can lead to incomplete transfer if the time that the image-bearing sheet and the receiver member are in transfer relation is not of sufficient duration.